IV Decompression Theory A. Introduction Following the commencement of the industrial revolution it became possible for human beings to manipulate the physics and chemistry of their environment on a grand scale. This led to the appearance of a large number of man-made disorders, for many of which the causative agent is easily established; the mechanism whereby the body reacts to these agents is, however, not sufficiently understood. Decompression sickness is such a disorder. It is provoked by our ability to change the pressure and chemical composition of the gases we breathe. One procedure for complete prevention of decompression sickness is therefore to not venture outside the limits of air composition and partial pressures normally encountered on the surface of the earth. A second way to eliminate the harmful effects of decompression sickness is to understand the responsible mechanisms and from this understanding to con­ struct safe procedures. It is, of course, this second possibility that is examined in this chapter. The fIrst successful pump for exhausting the air from a container was invented in the 17th century by Von Guericke. Using his own version of this recently invented pump, Robert Boyle became in 1670 the fIrst investigator in the fIeld of decompression sickness when he decompressed a viper in his "exhausted receiver" and described the now-famous "bubble moving to and fro in the waterish humour of one of its eyes." From this and similar early experiments the idea arose that a rapid reduction of atmospheric pressure could lead to the release of bubbles into sensitive tissues of the body and that this could seriously impair their normal functions. For work underwater, gases at pressures greater than atmospheric pressure are re­ quired, and it was some years before a pump could be used to raise the ambient pressure rather than lower it, as in the Boyle experiments described above. Many attempts had been made to descend into the sea by using a variety of diving apparatus, but until comparatively recent times all these devices were totally impractical for any reasonably prolonged under­ water work and offered very little advantage beyond simple breath-hold diving. The diving bell was really the first successful underwater device, and it is generally agreed that Ed­ mund Halley (of comet fame) designed, built, and used the first practical "bell' system 223 C. W. Shilling et al. (eds.), The Physician’s Guide to Diving Medicine © Plenum Press, New York 1984 224 Chapter IV (Halley 1717). An artist's impression of this bell is given in Figure IV-I, and a brief description of this device serves to illustrate some of the difficulties facing the theoretician when he attempts to collect reliable data as a basis for his ideas. Halley's bell was con­ structed of wood and lined on the exterior surface with lead sheeting in order to give sufficient weight and stability to the bell when underwater. It had a cubic capacity of nearly 60 ft 3 (I. 7 m3) and was approximately 3 ft (0.9 m) in diameter at the top. It was realized that with two men in the bell when it was immersed in water the air contained within the bell would become foul. Drawing on more recent knowledge one can easily calculate that Figure IV-I. Artist's conception of Edmund Halley's diving bell (ca. 1717). [Adapted from Davis (1981).] Decompression Theory 225 with two active men inside a bell the carbon dioxide concentration would reach about 3% in I hr, and this would be the useful duration for such a bell without some form of air replenishment. Fresh air, in this case, was supplied to the bell from lead-lined barrels having bung holes at the top and the bottom and a leather tube through which air could be forced from the barrels into the bell. After being emptied of air these barrels were hauled to the surface, where their air content was renewed, and thus the whole process was a continuous one. Depths as great as 60 ft (18 m) for dive durations as long as I 1/2 hr were attained by using this particular technique. Several important physiological points relevant to the theoretician should be noted. First, the nature of the breathing gas must be clearly defined; variability in the concentration of such physiologically active gases as carbon dioxide and oxygen could influence the validity of any decompression observations made. Second, the diver who leaves the bell at the end of his breathing tube is being subjected to a different environment from that of his companion seated within the bell. The diver may well be surrounded by quite cold water with the well-known ensuing physiological reactions to this. In addition he is attempting to draw fresh air for breathing purposes down a tube that is clearly giving added respiratory airflow "drag." He is rebreathing expired air. His head, neck, shoulders, and particularly his respiratory passages are at risk of squeeze effects if his helmet is attached to his suit. If his helmet is open ended and not equipped with a nonreturn valve he is in danger of drowning upon any lowering of his helmet below the water level of the bell. (In contrast, any raising of his helmet above the bell's water level will produce a free flow of air into the helmet, with relief from the foul air and possible squeeze effects.) And with immersion the hydrostatic relationships between blood pressure in the extremities and the pressure in the heart are altered and thus there are changes in the cardiovascular system. Third, some underwater workers are likely to be performing very little work (e.g., the seated attendant, who mayor may not be pumping air into the diver's own hose by means of bellows), but others (e.g., the diver) are required to perform quite hard physical work. Fourth, human beings are very different in important factors such as stature, body composition, and level of physical fitness. Finally, and of extreme importance in studying the effects of exposure to raised pressures of air and other gases, it is necessary to know how long the subjects were exposed and to what pressure. With such a formidable list of variables to be brought under control in order to obtain consistent findings, it is hardly surprising that numerous and conflicting conclusions were reached from the data available in the early years of this discipline. Placing human beings under raised pressures of air can conveniently be considered divisible into four separate phases, each of which has its own particular set of problems. The first phase is taking the person to pressure, and this compression phase sometimes causes the establishment of pressure differentials in body cavities such as the sinuses and the middle ear, producing in these instances sinus and ear pain and vertigo. The second phase comprises the sojourn at full pressure. Here, the compressed air worker (diver, caisson, tunnel) encounters the effects of raised pressures of oxygen, carbon dioxide, toxic gas contaminants such as carbon monoxide, and the inert gas nitrogen, any or all of which can give rise to numerous difficulties (e.g., oxygen toxicity, nitrogen narcosis, carbon dioxide intoxication, carbon monoxide toxicity), altered thermal balance, increased respi­ ratory work, and communications problems. The third, or decompression, phase is the return to atmospheric pressure, which is followed by the fourth phase, or post-decompres­ sion period. It is these two latter phases that principally concern us. A note of caution must 226 Chapter IV be introduced because, as becomes apparent later, the events occurring in phase 2 can profoundly influence the body's responses in the decompression and post-<iecompression periods. For the moment, however, let us consider only the evidence accumulated by the turn of the century concerning decompression and its consequences. It had become apparent that decompression could be followed by harmful effects varying in severity from death to mild itching of the skin. The prevention of these ill effects was soon seen to lie in pursuing a slow release of pressure. It would seem that everyone adopted some form of linear de­ compression procedure, i.e., the pressure was released at a certain number of bar (psi, kg! cm2) per minute for the caisson and tunnel workers or a given number of meters (ft, fath­ oms) of ascent per minute for the diver. The practical problem in those days was to decide the most effective rate of pressure release. B. Defining the Problem An understanding of the basic processes producing the harmful effects of decompres­ sion was lacking until the time of Paul Bert (1878), who made numerous fundamental observations in a series of experiments between 1870 and 1890. He showed that the more serious forms of decompression sickness were provoked by the presence of large volumes of free gas, as opposed to dissolved gas, within body tissue. Furthermore, after careful analysis of the composition of these bubbles he concluded that nitrogen gas was the main constituent. Thus an outline picture of the etiology of decompression sickness could now be attempted: It is apparently caused by the release of gas emboli from nitrogen gas dis­ solved at pressure, and these nitrogen gas emboli then impair the functioning of the various tissues in which they lodge or are formed. Meantime, a clearer clinical picture was also emerging. If the decompression was grossly inadequate, then the blood literally "frothed" and a condition descriptively termed the chokes was encountered, and this proved rapidly fatal unless promptly treated by re­ compression. If the decompression was not so provocative, then a condition known as the staggers was often seen; this too was a very serious manifestation of decompression sick­ ness that could lead to permanent damage in the central nervous system, or even death.